Abstract
Flavonoids are a group of polyphenolic compounds widely present in the herbal world and playing an important role in the human diet. The flavanol quercetin makes up 70% of the total daily intake of flavonoids. Quercetin is able to complex with many metal ions, and it exhibits potent antioxidative ability. Using the equimolar solution variation method, it was confirmed that quercetin makes a complex with the zinc(II) ion at pH 5.25, in a stoichiometric ratio quercetin:zinc(II) ion = 2:1, with the absorption maximum l=363 nm. This fact was used to develop a simple, precise and accurate assay to determine the content of quercetin in various samples of heterogeneous composition. The proposed indirect spectrophotometric method can selectively determine quercetin in concentrations ranging from 0.1 to 6.0 mgL-1, with LOD and LOQ estimated as 0.03 mgL-1 and 0.1 mgL-1, respectively. The reliability of the proposed method was confirmed by a previously developed RP-HPLC/UV method. The proposed method was successfully used to determine the quercetin content in dietary supplement tablets, capsules and two onion extracts, with high reproducibility. The antioxidative ability of quercetin and the zinc(II)-quercetin complex was determined using DPPH and FRAP methods. The same samples were tested for antimicrobial activity against seven laboratory control strains of bacteria and one strain of yeast. As a result of those tests, there are no obstacles to combine quercetin and zinc in the same supplement formulation.
Highlights
In the present development stage of the world population, it is necessary to recognize the health benefits and potential of agricultural products regularly present in the human diet
We have developed spectrofluorimetric and spectrophotometric methods based on the complexes formed with metal ions, and successfully applied them for the determinations of several flavonoids in fruit juices and/or pharmaceutical dosage forms (Pavun et al, 2018, Pavun et al, 2016, Pavun et al, 2014)
The proposed spectrophotometric method for the determination of quercetin based on the formation of the Zn2+ ionquercetin complex proved to be reproducible and accurate, with a high value of the coefficient of determination r2 = 0.99992
Summary
In the present development stage of the world population, it is necessary to recognize the health benefits and potential of agricultural products regularly present in the human diet. Quercetin (Fig. 1) is a flavonoid (plant pigment) commonly found in fruits and vegetables, especially onions, citrus and apples. Green tea and red wine have been pointed out as having notable amounts of quercetin. The primary benefit of quercetin is that it possesses potent antioxidant properties. Antioxidants fight against free radicals – chemically reactive compounds that damage cell membranes and DNA and may cause cell death. Many of the benefits that are often attributed to antioxidants refer to the effects related to quercetin. Quercetin possesses antiinflammatory and anti-allergic abilities, realized through the inhibition of lipoxygenase and cyclooxygenase effects (Li et al, 2016)
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